Provenance and geochemistry of exotic clasts in conglomerates of the Oligocene Torehina Formation, Coromandel Peninsula, New Zealand

Abstract

Non-marine pebble to cobble conglomerates of the lower Torehina Formation (Oligocene) crop out along western Coromandel Peninsula and overlie, with strong angular discordance, continental-margin metasedimentary rocks (Manaia Hill Group) of Mesozoic (Late Jurassic to ?Early Cretaceous) age. The conglomerates contain provenance information that identifies a pre-Oligocene depositional history obscured by the unconformable juxtaposition of these Tertiary and Mesozoic strata.Most clasts in the lower Torehina Formation are visually similar to local bedrock lithologies, including metamorphosed sandstones and argillites, but are kaolinitic and contain more detrital and authigenic chert, quartz, and potash feldspar. Local derivation of these clasts seems unlikely. By comparing geochemical ratios with those defined for continental margin sandstones, and well characterised New Zealand tectonic terranes, we interpret the majority of clasts in the lower Torehina Formation to have been derived from a dissected orogen, with mixtures of felsic and volcanogenic-derived sediment. The most likely sources are the Waipapa and Torlesse Terranes.The remaining 20-30% of the clasts in the lower Torehina Formation were originally friable, are coarse grained, and appear to be lithologically exotic relative to known metamorphosed sandstones in basement terrane sources on North Island. Some clasts contain coal laminae and particles, and all contain detrital kaolinite as lithic fragments and matrix. Such characteristics imply a non-marine to marginalmarine source containing sediment derived from strongly weathered granite or granodiorite. Mechanical fragility implies a likely proximal, easily erodible source. We propose that this group of clasts was derived from an Upper Cretaceous sedimentary cover, either part of a locally developed basin fill or part of a once regionally extensive cover on North Island. Either case defines a more widely distributed Cretaceous source than found today.

The Waipounamu Erosion Surface: questioning the antiquity of the New Zealand land surface and terrestrial fauna and flora

“…A prominent intraformational erosion surface separates the lower shelf clastic sediment and limestone from overlying deepwater Oligocene-Early Miocene limestone (Dix & Nelson, 2004). This surface, interpreted as a sequence boundary by Dix & Nelson (2004), separates a highly variable, carbonate-dominated, transgressive basal marine sequence from overlying more uniform, and slightly less steeply dipping, deep sea carbonate sequence. Limestone-forming conditions persisted from Late Oligocene (24 Ma) to Early Miocene time (21 Ma).…”

“…1), Neogene erosion and volcanism have removed or buried much of an inferred widespread earlier Cenozoic cover sequence. However, isolated remnants record a regional transgression beginning in the Eocene and peaking in the earliest Miocene (Nelson, 1978;Dix & Nelson, 2004).…”

“…1) shows a basal unconformity overlain by fandelta sediments of Early to middle Oligocene age that are in turn overlain by younger Oligocene shallow marine limestone (Dix & Nelson, 2004). A prominent intraformational erosion surface separates the lower shelf clastic sediment and limestone from overlying deepwater Oligocene-Early Miocene limestone (Dix & Nelson, 2004).…”

“…1) shows a basal unconformity overlain by fandelta sediments of Early to middle Oligocene age that are in turn overlain by younger Oligocene shallow marine limestone (Dix & Nelson, 2004). A prominent intraformational erosion surface separates the lower shelf clastic sediment and limestone from overlying deepwater Oligocene-Early Miocene limestone (Dix & Nelson, 2004). This surface, interpreted as a sequence boundary by Dix & Nelson (2004), separates a highly variable, carbonate-dominated, transgressive basal marine sequence from overlying more uniform, and slightly less steeply dipping, deep sea carbonate sequence.…”

The role of tectonism in sequence development and facies distribution of Upper Oligocene cool‐water carbonates: Coromandel Peninsula, New Zealand

“…The majority of clasts in F1A conglomerates are lithologically similar to those associated with local and nearby Mesozoic basement rocks (Dix & Nelson, 2003). However, some friable clasts were derived from a probable wider distribution of Upper Cretaceous sandstones than that now found on North Island (Dix & Nelson, 2003). F1B sandstones have a similar lithology to these clasts (Fig.…”